The present invention relates to bearing supports for a main bearing in an internal combustion engine.
Mass reduction is a major goal in engine design. Therefore it is known to substitute aluminum for iron in certain engine components. The drawback in some applications is that commonly employed aluminum alloys, such as 380 aluminum, have a much higher coefficient of thermal expansion (CTE) than iron. For example, cast iron has a CTE of about 12xc3x9710xe2x88x926/K, whereas 380 aluminum has a CTE of about 21xc3x9710xe2x88x926/K, a factor of almost two. Therefore in the case where aluminum bearing caps support an iron crankshaft, the aluminum bearing caps thermally grow at a faster rate than the crankshaft, resulting in an increased bearing clearance and potentially unacceptable noise generation. Greater bore clearances require larger capacity lubrication systems to compensate for oil leakage past the main bearings and to maintain adequate oil film thickness on the bearings.
One solution to the thermal expansion issue, as described in U.S. Pat. No. 5,203,854, is to produce an aluminum bearing cap cast with an iron core adjacent to the crankshaft bore to provide comparable coefficients of thermal expansion between the crankshaft and the bearing cap. It is proposed that the bearing clearance does not significantly vary and therefore noise generation is reduced.
One obstacle with casting an iron core in an aluminum bearing is to ensure a high strength connection between the iron core and the aluminum casting. U.S. Pat. No. 5,203,854 addresses the concern of boring dissimilar materials, in particular a semi-circular iron surface in the bearing cap and a complementary semi-circular surface in an aluminum engine block. Due to the different hardnesses, tool selection and settings may be compromised, which could potentially affect the quality of the crankshaft bore. To facilitate machinability, an aluminum layer of a few millimeters in thickness is added to the bore surface so that the boring tool cuts the same material throughout the revolution.
The purpose of the present invention is to provide a main bearing support which has a CTE comparable to the crankshaft it supports, has machining characteristics similar to the complementary boring surface, and is comprised of a single alloy for efficient recyclability.
The present invention is for an internal combustion engine having an aluminum engine block, an iron crankshaft, and main bearing supports comprised entirely of an aluminum-silicon alloy. The aluminum-silicon alloy has the following material characteristics: low CTE comparable to iron alloy; high strength comparable to iron alloy; and good machinability similar to other aluminum alloys such as 380 aluminum. The hypereutetic aluminum-silicon alloy that satisfies these characteristics is composed of at least 25% silicon by weight with the remaining being aluminum.
More specifically, aluminum-silicon alloy with 33-35% silicon has a CTE of approximately 13.2xc3x9710xe2x88x926/K, which is comparable to iron alloy. Previously such a low CTE was not achievable with aluminum alloys. This material selection ensures that a tighter crankshaft bore tolerance may be maintained through consistent thermal rates of expansion of the iron crankshaft and aluminum-silicon bearing supports. A tighter crankshaft bore tolerance will increase bearing life as there is less wear of the bearings.
The use of an aluminum-silicon bearing support in conjunction with an aluminum engine block provides ease of machining the crankshaft bore; it eliminates mixed material machining previously required between iron bearing supports and an aluminum engine block. Aluminum machines much faster and easier than iron thereby reducing machining cycle time and tool wear.
Therefore main bearing supports composed of such a hypereutetic aluminum-silicon alloy provide a combination of low mass, ease of machining, and dimensional stability of the crankshaft bore, all with a single alloy.